Organic EL devices are a class of self-emission devices that make use of the principle of electroluminescence—when an electric field is applied, a fluorescent material emits light because of the energy released from the recombination of electrons and holes injected from the cathode and anode, respectively. C. W. Tang et al. of Eastman Kodak Company reported a low-voltage-driven organic EL device using a double layered structure (C. W. Tang, S. A. Vanslyke, Applied Physics Letters, Vol. 51, 913 (1987), etc.). Since then, extensive research has been undertaken on organic EL devices that utilize organic EL materials. Well-known device structures for organic EL devices include a double layer structure composed of a hole transport (injection) layer and an electron transport/emitting layer; and a triple layer structure composed of a hole transport (injection) layer, an emitting layer, and an electron transport (injection) layer. Aiming to increase the recombination efficiency of injected holes and electrons, various improvements have been made on the device structure and fabrication process of such stack-type organic EL devices.
Driving or storing organic EL devices under high temperature conditions generally leads to adverse consequences, including changes in emitted light color, reduced luminous efficiency, elevated driving voltage, and shorter emission lifetime. To avoid these drawbacks it has been required in the art to increase the glass transition temperatures (Tg) of hole transport materials, requiring molecules of hole transport materials to have many aromatic groups (see, e.g., aromatic diamine derivatives disclosed in Patent Literature 1 and aromatic condensed ring diamine derivatives disclosed in Patent Literature 2). Typically, hole transport materials that have 8 to 12 benzene rings per molecule are preferably used.
However, manufacturing of an organic EL device by forming a thin film of hole transport material that has many aromatic groups in its molecule has encountered problems, such as clogging of the opening of the vapor deposition crucible by the crystallized material, and reduction in manufacturing yield caused by defects in the deposited thin film due to crystallization. Compounds that have many aromatic groups in their molecules generally have high glass transition temperatures (Tg), but at the same time have high sublimation temperatures, which is considered to be responsible for the material decomposition during vapor evaporation, non-uniform thickness in the deposited films and other phenomena, leading to shorter device life.
Meanwhile, as hole transport materials, carbazole-containing monoamine derivatives are also known (see Patent Literatures 3 to 7). These compounds, however, cannot sufficiently prolong the life and improve the luminous efficiency of organic EL devices, especially after high temperature storage. Additionally, amine derivatives are known in which carbazole and amine are linked together with fluorene (see Patent Literatures 8 and 9), but the effects of prolonging life and improving luminous efficiency have likewise been needed for these compounds.
Although some hole transport materials for realizing long-life, high-efficient organic EL devices have been reported so far as described above, there is strong demand for device materials that can provide organic EL devices with more excellent performance.